The invention relates generally to turbomachinery and, more specifically, to the axial stabilization of airfoils therein.
Axial flow turbomachinery is generally constructed with one or more airfoil or blade rows being rotatably disposed in close radial relationship within an outer shroud or casing. In the case of a turbine, the motive gases flowing axially within the casing, perform work to rotate the airfoils, and in the case of a compressor or fan, the gases flowing within the casing are compressed by the airfoils or blades performing work on the gas. In either case, it is desired for the sake of efficiency to have the tips of the airfoils be as closely spaced to the casing as possible in order to reduce the amount of leakage of air therebetween. Of course, it is also necessary to prevent any substantial interference between the rotating airfoils and the stationary casing.
One approach, which has been commonly used, is that of placing an abradable material, such as honeycomb or the like, within the casing surrounding the airfoils and establishing a slight clearance between those components. With mechanical growth, the airfoils will then tend to slightly wear into or cut a groove into the abradable material to thereby establish a sealing relationship therebetween. However, even after a groove has been formed, because of the relative mechanical and thermal growth patterns between the rotating airfoils and the stationary casing, there will still be periodic rubs occurring, which in turn will transmit resistant forces to the airfoils or fan blades. Since the airfoils are necessarily twisted in order to compress the air when rotated, the blade tips are also disposed at an angle relative to the axis of rotation. Thus, where a rub occurs, there is transmitted to the blades a resultant force which has both tangential and axial components. To the extent that the rub forces are axial in direction, they tend to force the blades axially from their dovetail slots. If the rubs are severe enough, these axial forces may cause the blades to actually be forced from their dovetail positions to thereby cause severe damage to the turbomachine.
It is, therefore, an object of the present invention to reduce the axial rub forces acting on an airfoil.
Another object of the present invention is the provision in an airfoil/casing relationship for reducing the axial component of the reactive forces acting on the airfoil when it rubs against the surrounding casing.
Yet another object of the present invention is to maintain close clearances between an airfoil rotor and a surrounding casing without attendant relative axial forces.
Still another object of the present invention is the provision for an airfoil rotor and surrounding casing combination which are economical to manufacture and effective in use.
These objects and other features and advantages become more readily apparent upon reference to the following description when taken in conjunction with the appended drawings.